Copolymer of Styrene and Maleic Anhydride Comprising an Epoxy Resin Composition and a Co-Cross-Linking Agent

ABSTRACT

Laminates for printed wiring boards for the making of laminates for printed wiring boards having an impregnant including an epoxy resin, a first cross-linking agent of a strene-maleic anhydride copolymer and a second co-cross-linking agent.

The invention pertains to a resin composition comprising a styrenemaleic anhydride copolymer as a cross-linking agent (curing agent), anepoxy resin, and a co-cross-linking agent.

The use of cross-linking agents for epoxy resin is described in BE627,887. This patent also discloses a proposal to use copolymers ofmaleic anhydride and styrene (SMA) as cross-linking agent for epoxyresin. A drawback to such epoxy resin compositions is that they have lowTg and low thermal stability, rendering them unsuitable for use inprepregs, which are applied in laminates for printed wiring boards(PWBs).

The resin generally used in electrolaminates is an epoxy resin. Thepresent practical standard is the FR4-laminate, which is based on abrominated epoxy resin prepared from a diglycidyl ether of bisphenol-Aand tetrabromo-bisphenol-A, dicyanodiamide as curing agent, an organicsolvent, and an accelerator. The drawback to such an epoxy resin is itslow Tg (110-130° C.), while in addition the dicyanodiamide has atendency to crystallize in the resin and the prepreg made therefrom.

An improvement has been sought in the preparation of an interpenetratingpolymeric network (IPN). Such resin compositions are known from EP413,386. This document relates to IPNs having very favorable properties,in particular for use in the electronics industry. This is the case whenthe cross-linking agent used for the epoxy resin is a polybrominatedphenol. In actual practice, the embodiment using anhydride cross-linkingagents proves unsatisfactory. Notably, the Tg obtained is too low, andthe electrical properties and the prepreg stability also leave room forimprovement.

In addition, it is desired that the use of inexpensive difunctionalepoxy resins should give thermal properties which are of the samestandard as can be obtained using the multifunctional epoxy resinspreferably employed in EP 413,386. Resins based on multifunctional epoxycompounds have been described in WO 85/03515 and WO 86/02085.

Other publications describing epoxy resin compositions employinganhydrides as cross-linking agent for the epoxy resin are U.S. Pat. No.2,707,177; DE 3,521,506; GB 994,484; and EP 417,837. This last patentspecification teaches the use of ethylenically unsaturated anhydrides,such as maleic anhydride, where the anhydride not only cross-links theepoxy resin but also takes part in the forming of the network.

A solution to the above-mentioned problems has been suggested in WO96/07683, which discloses a resin composition where the carboxylicanhydride is a copolymer of an ethylenically unsaturated anhydride and avinyl compound. In such a copolymer the ethylenically unsaturatedportion of the anhydride is incorporated into the backbone. Thecarboxylic anhydride groups remain intact, and they are available asfunctional groups for cross-linking the epoxy resin. More specifically,those resin compositions contain triallyl cyanurate (TAC) as allylpolymerizing agent. In this type of resin TAC is necessary to obtaincompositions with high Tg and acceptable thermal stability, which can beapplied in prepregs.

Resin compositions which comprise dicyandiamid as co-cross-linking agenthave been disclosed in DE 38,39,105. According to this document,dicyanamid is an essential constituent of the resin composition.Dicyandiamid, however, has the disadvantage that it only dissolves intoxic and expensive solvents, and it would be an advantage to findsuitable co-cross-linking agents, devoid of the disadvantages ofdicyandiamid.

Epoxy resin compositions containing low-molecular weight copolymers ofα-methylstyrene and maleic anhydride have been disclosed in U.S. Pat.No. 4,042,550. Such compositions are unsuitable for the manufacture ofPWBs.

Prepregs are widely employed in the manufacture of laminates for theelectronics industry, in particular for printed wiring boards. Suchmanufacture-involves impregnating a supporting or reinforcing fabricwith a resin, followed by partial curing of said resin. Such impregnatedfabric is commonly referred to as prepreg. Manufacturing a printedwiring board involves laminating one or more layers of prepreg with,say, one or more layers of copper.

Processing prepregs into boards usually involves their being cut down tosize and laminated. Both these process steps make stringent demands onthe resin with which the fabric is impregnated. For instance, thepartially cured resin has to have sufficient sturdiness and a highviscosity, yet it must be sufficiently sticky and liquid to give goodadhesion when laminated, and hence good interlaminar strength. The resinmay not be too highly reactive, since this will render the requiredpartial curing impossible.

In this connection resin compositions where the epoxy resin iscross-linked with an anhydride-containing copolymer have the drawback ofbeing too brittle to be processed as prepregs. For instance, it provesimpossible to cut up such prepregs without a portion of the resinblowing about in the form of a large quantity of dry dust. This issometimes called a “mushroom effect,” after mushroom spores blowingabout.

It has now been found that in contrast to the previous solutions forobtaining suitable polymers for use in prepregs, IPNs are not necessary,and that epoxy resins free from TAC can be prepared having high Tgand/or improved thermal stability.

On the one hand, the invention has the object to enhance the thermal andelectrical properties of resin compositions based on epoxy resincross-linked with styrene maleic anhydride copolymer (SMA). On the otherhand, the invention envisages resin compositions based on difunctionalepoxy resin which have thermal and electrical properties comparable toIPNs the resin composition of which is based on multifunctional epoxycompounds. Furthermore, the invention aims to provide resin compositionswhere the problem of brittleness, which occurs when SMA is used as epoxycross-linking, agent, can be prevented.

To this end, the invention consists of a resin composition comprising acopolymer of styrene and maleic anhydride (SMA) as cross-linking agent,an epoxy resin, and a co-cross-linking agent, characterized in that theco-cross-linking agent is an optionally brominated bisphenol A (BPA), oran optionally brominated bisphenol A diglycidyl ether (BPADGE), or amixture thereof, and that the composition is free from an allyl networkforming compound.

It is known from WO 96/07683 that epoxy resin compositions which arefree from allyl network forming compound, such as TAC, have low Tg,usually not higher than 130° C., and low thermal stability. The presentinvention is based on the finding that use of BPA as co-cross-linkingagent significantly improves the thermal stability of the co-polymer. Tothis same end it was found that BPADGE as co-cross-linking agentconsiderably increases the Tg, so that Tg values of 190° C. areattainable.

Preferably, the co-cross-linking agent is a brominated BPA, a brominatedBPADGE, or a mixture thereof. More preferably, the co-cross-linkingagent is tetrabromobisphenol A (TBBPA) or tetrabromobisphenol Adiglycidyl ether (TBBPADGE). Most preferably, the co-cross-linking agentis a mixture of tetrabromobisphenol A (TBBPA) and tetrabromobisphenol Adiglycidyl ether, leading to resin compositions with high thermalstability and a high Tg. Moreover, the stability of prepregs made of theresin composition of the invention is considerably improved with respectto the prior art prepregs. A further advantage is that post-curing,which is necessary with IPNs, is not longer required.

Copolymers of styrene and maleic anhydride have been described, interalia, in Encyclopedia of Polymer Science and Engineering Vol. 9 (1987),page 225 ff. Within the framework of the invention the term “copolymer”likewise refers to SMA or mixtures of SMA.

Copolymers of styrene and maleic anhydrides (SMA) are commerciallyavailable in two types. Type 2 comprises mostly high-molecular weightcopolymers (MW generally higher than 100,000, for instance, 1,000,000).These are in fact thermoplasts, which are unsuitable for use in themanufacture of prepregs. Moreover, because of their low anhydridecontent (5-15%) they are not particularly suitable for use as across-linking agent for epoxy resin either. The type 1 SMA copolymers,on the other hand, which have a molecular weight in the range of about1400 to about 50,000 and an anhydride content of more than 15% byweight, are pre-eminently suited to be used. Preference is also given toSMA copolymers having a molecular weight in the range of 1400 to 10,000.Examples of such copolymers, include the commercially available SMA1000, SMA 2000, SMA 3000, and SMA 4000. These copolymers have astyrene:maleic anhydride ratio of 1:1, 2:1, 3:1, and 4:1, respectively,and a molecular weight ranging from about 1400 to about 2000. Mixturesof these SMAs may also be used.

The amount of copolymer employed can be such as will give an anhydrideand aromatic hydroxy groups: epoxy groups equivalency ratio in the rangeof 50 to 150% by weight. The preferred ratio is between 75 and 125% byweight, and more preferably between 90 and 110% by weight. Optimumresults are obtained when at least 10% by weight of TBBPA and at least10% by weight of TBBPADGE are employed as co-cross-linking agents.

The term “epoxy resin” in this context refers to a curable compositionof oxirane ring-containing compounds as described in C. A May, EpoxyResins, 2^(nd) Edition, (New York & Basle: Marcel Dekker Inc.), 1988.

Examples of epoxy resins include phenol types such as those based on thediglycidyl ether of bisphenol A, on polyglycidyl ethers ofphenol-formaldehyde novolac or cresol-formaldehyde novolac, on thetriglycidyl ether of tris(p-hydroxyphenol)methane, or on thetetraglycidyl ether of tetraphenylethane; amine types such as thosebased on tetraglycidyl-methylenedianiline or on the triglycidyl ether ofp-aminoglycol; cycloaliphatic types such as those based on3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. The term“epoxy resin” also stands for reaction products of compounds containingan excess of epoxy (for instance, of the aforementioned types) andaromatic dihydroxy compounds. These compounds may behalogen-substituted.

Preference is given to epoxy-resins which are derivative of bisphenol A,particularly FR4, especially on account of their low price. FR4 is madeby an advancing reaction of an excess of bisphenol A diglydicyl etherwith tetrabromobisphenol A. Mixtures of epoxy resins with bismaleimideresin, cyanate resin and/or bismaleimide triazine resin can also beapplied.

It should be noted that epoxy resins are generally represented by asingle, unequivocal structural formula. The skilled person will knowthat this should be taken to include deviating products resulting fromside reactions occurring during epoxy resin preparation. As these sideproducts constitute a normal component of cured epoxy resins, theylikewise constitute a normal component of the resins according to theinvention.

The BPA and BPADGE may optionally be brominated, i.e. substituted withone or more bromine atoms. Brominated co-cross-linking agents arepreferred because of their flame retarding properties. Preferably, thearomatic moieties of both BPA and BPADGE are substituted with twobromine atoms, to give tetrabromo substituted TBBPA and TBBPADGE,respectively. Optionally brominated novolacs can also be used asco-cross-linking agent.

Cross-linking of the epoxy resin generally proceeds with the aid of anaccelerator. As suitable accelerators may be mentioned imidazoles, moreparticularly alkyl substituted imidazoles such as 2-methylimidazole and2-ethyl-4-methylimidazole, and tertiary amines, e.g.benzyldimethylamine.

The amount used of such an accelerator is dependent on the type of epoxyresin, the type of cross-linking agent, and the type of accelerator.Employing a too large amount of accelerator will lead to a too highlyreactive resin system. Such a system is not serviceable for makingprepregs. The skilled person can easily determine within which range aresin system will be just sufficiently little reactive to allow readyprocessing into prepregs. In general, such a processing range will bebetween 0.01 and 5% by weight of accelerator, calculated on the overallweight of epoxy resin and cross-linking agent. In many cases this willbe the 0.01-0.075% by weight range. The gel time for its part isdependent on the type and amount of accelerator, the type and amount ofsolvent, and the type of prepreg to be manufactured. In the specificcase of 2-methylimidazole (2MI) being used as accelerator, it ispreferred not to use more than about 0.05% by weight of 2MI. By way ofgeneral guideline it can be said that it is advisable not to have avarnish gel time of less than 120 seconds.

The desired resin properties determine the amount of BPA and BPADGE tobe incorporated into the resin. According to the invention, forinstance, it has surprisingly been found that the Tg of epoxy resinscross-linked with SMA can be increased substantially by the use of atleast 5% by weight of BPA. Most surprisingly of all, it is now possible,as indicated above, to obtain resins having glass transitiontemperatures of 130° C. and higher even with simple difunctional epoxycompounds.

As a rule, an organic solvent is employed when preparing resinsaccording to the invention. If a solvent is used, it must be one inwhich the epoxy resin, cross-linking agent, and co-cross-linking agentare soluble, while the solvent itself should be sufficiently volatile toevaporate before or during the curing.

As suitable solvents may be mentioned dimethylformamide; glycol etherssuch as ethylene glycol mono-ethyl ether or propylene glycol mono-ethylether and their esters such as ethylene glycol mono-ethyl ether acetate;ketones such as methyl isobutyl ketone, methylethyl ketone, acetone, andmethyl isopropyl ketone; and aromatic hydrocarbons such as toluene andxylene. Alternatively, mixtures of solvents can be employed. Thepreferred solvents are ketones, notably acetone and methylethyl ketone,or mixtures of these with ethers, notably propylene glycol mono-ethylether.

The invention further pertains to laminates for use in the electronicsindustry incorporating resins of the aforementioned type.

Laminates for use in the electronics industry (particularly for printedwiring boards) are generally produced by impregnating a supporting orreinforcing material (usually based on glass fibres, either as a wovenfabric or in the form of a cross-ply laminate of unidirectionallyoriented parallel filaments) with a resin, followed by the resin beingcured wholly or in part. The latter process is the most common one, anda fabric impregnated with a partially cured resin is usually referred toas a “prepreg.” To make a printed wiring board from a prepreg fabric oneor more layers of the prepreg are laminated with, say, one or morelayers of copper.

The resins according to the invention are highly suitable forimpregnating, e.g., woven fabric and cloth of a variety of materialssuch as glass, quartz, carbon, aramid, and boron fibres, moreparticularly to make laminates for printed wiring boards. Thisapplication preferably calls for the resin to be employed in combinationwith a glass fabric.

It was found that even when it is based on simple difunctional epoxycompounds, the combination of resin components according to theinvention will give excellent properties for application in theelectronics industry. The Tg effect has been mentioned earlier: ascompared with the corresponding standard epoxy resins (cured withdicyanodiamide) the resins according to the invention have a Tg of about30-50° C. higher. Furthermore, it was found that resins according to theinvention exhibit a much better resistance to short, intensetemperature, increases than do standard FR4 epoxy resin and IPNsaccording to EP 413,386 and have better prepreg stability. The thermalstability is demonstrated by the pressure cooker test and the soldershock test, which are known to the skilled man. The pressure cooker testis a procedure for evaluating glass epoxy laminate integrity. In thistest a specimen of the laminate to be tested is placed in a pressurecooker for some time, after which the specimen is immersed in a solderbath at 260° C. The specimen is thereafter graded on the occurrence ofmeasles, blisters, delamination, convolution, and surface erosion. Thelonger the cooker time without said occurrence is, the more thermallystable the laminate will be. In the solder shock test a material istransferred abruptly from room temperature to solder having atemperature of 288° C. The material (in this case a laminate made of aresin according to the invention) floats in the solder, and so will besubject to a temperature gradient (and hence a tension gradient). Thematerial should be capable of withstanding these conditions for at least30 seconds without bubble formation or delamination occurring. Thelonger the material can stand the test, the more serviceable it will befor use in printed wiring boards. The resins according to the inventionare capable of standing the solder shock test for 10 minutes, whichrepresents a substantial improvement over both the aforementioned knownIP Ns, which bear it for about 3 minutes, and FR4 epoxy resin (about 4minutes). Furthermore, the resins according to the invention exhibit asignificant reduction of dielectric loss.

Also, the resins according to the invention can be employed wherever useis made of conventional epoxy resins: as a glue, coating, molding resin,embedding resin, encapsulating resin, sheet molding compound, bulkmolding compound.

In addition to being used as composites for printed wiring boards, theresins according to the invention can be employed to make compositesfor, inter alia, the construction, aviation, and automobile industries.The manufacture of appropriate structural composites may proceed in aknown manner, e.g., by impregnating reinforcing material with molten ordissolved resin, or via resin transfer molding, filament winding,pultrusion, or RIM (reaction injection molding).

The resins according to the invention may contain the usual additivessuch as dyes or pigments, thixotropic agents, fluidity control agents,and stabilizers.

The invention will be further illustrated with reference to thefollowing examples.

EXAMPLE 1

In a typical example 925 g of BPADGE (DER 535 EK 80) were mixed understirring with methylethyl ketone (MEK) to a 80% solution. To thissolution were added subsequently 1560 g of SMA 3000 as a 50% solution inMEK, 200 g of TBBPA, 280 g of TBBPADGE (Quatrex 6410), 400 g of MEK, and8 g of a 10% solution of accelerator (2-methylimidazole) inmethoxypropanol. The concentration of the accelerator is 0.04% relativeto the solid contents of the complete resin. The equivalency ratio ofanhydride and aromatic hydroxy groups: epoxy groups is 125%. The resinwas stirred for 1 h, after which a gel time of 200 sec at 171° C. wasmeasured. The resin solution was fed into the trough of the impregnateddevice. The glass web (style no. 7628) was continuously drawn throughthe trough and through the heating zone. In the heating zone the solventwas evaporated and the resin was partially cured. The prepreg was runthrough the cooling zone and then cut into sheets. A standard lay-up wasprepared from 8 prepregs sheets and copper foil on both sides. Thelay-up was pressed to a laminate for 1 h at 1400 kPa and 171° C., andthe laminate was post-cured for 2 h at 200° C. This laminate proved tohave very good thermal properties with a Tg of 175° C., and an excellentsolder bath stability after 6 h in the pressure cooker.

EXAMPLE 2

The following resin compositions were prepared according toabove-mentioned proceedings (Example 1) (percentages by weight).

Pressure FR4 TBBPA TBBPADGE SMA Tg Cooker (%) (%) (%) (%) type (° C.)Test (h) 61 12 — 27 3000 149 6 66 13 — 21 2000 137 6 37 10 14 39 3000175 6 36 3 21 40 3000 176 3 30 — 27 43 3000 186 2 40 — 24 36 2000 186 356 — 17 27 1000 190 1

EXAMPLE 3

The Tg of laminates made from resin compositions having differentequivalency ratio (eq ratio) of anhydride and aromatic hydroxy groups:epoxy groups was determined by TMA and DSC. The laminates were made at aprepreging temperature of 171° C., pressed for 1 h at 171° C. and 1400kPa, and post-cured for 2 h at 200° C.

eq ratio TMA-Tg DSC-Tg mean value (%) (° C.) (° C.) (° C.) 70 116 122119 90 148 149 149 110 150 155 153 130 138 150 144 150 134 137 136

EXAMPLE 4

The Tg (mean value of TMA-Tg and DSC-Tg) of laminates made from resincompositions having different equivalency ratio (eq ratio) of anhydrideand aromatic hydroxy groups: epoxy groups was determined in relation tothe post-curing (PC) conditions. The results were compared with twoprior art TAC-containing laminates. The laminates were made at aprepreging temperature of 171° C., pressed for 1 h at 171° C. and 1400kPa, and post-cured as indicated.

eq Tg Tg Tg ratio (° C.) (° C.) (° C.) resin type (%) no PC PC: 185° C.;2 h PC: 200° C.; 2 h this invention 70 112 122 119 this invention 110149 155 153 this invention 130 140 146 144 this invention 150 136 138136 prior art resin A 90 169 171 181 prior art resin B 90 160 172 181

1-10. (canceled)
 11. A laminate made from a resin compositioncomprising: a mixture of: an epoxy resin that is a derivative ofbisphenol A; a crosslinking agent comprising a copolymer of styrene andmaleic anhydride (SMA), and; at least 5% by weight of a co-crosslinkingagent selected from the group consisting of a brominated bisphenol A(BPA), brominated bisphenol A diglycidyl ether (BPADGE), and mixturesthereof wherein The laminatedoes not include an acrylic resin.
 12. Thelaminate of claim 11 wherein the copolymer of styrene and maleicanhydride has a molecular weight of about 1400 to about 50,000, and ananhydride content of more than 15% by weight.
 13. The laminate of claim11 wherein the epoxy resin is FR4.
 14. The laminate of claim 11, whereinthe resin includes an accelerator.
 15. The laminate of claim 14 whereinthe accelerator is an imidazole.
 16. The laminate of claim 11 whereinthe resin includes one or more additives selected from the groupconsisting of dyes, pigments, thixotropic agents, fluidity controlagents, and stabilizers.
 17. The laminate of claim 11 wherein the resinhas a ratio of anhydride and aromatic hydroxyl groups: epoxy groups offrom 90 to
 130. 18. The laminate of claim 11 wherein the resin has aratio of anhydride and aromatic hydroxyl groups: epoxy groups of from 90to
 110. 19. The laminate of claim 11 wherein the resin is partiallycured.
 20. The laminate of claim 11 wherein the resin is fully cured.21. A printed circuit board including one or more laminate layerswherein at least one of the laminate layers is the laminate of claim 11.22. A laminate made from a resin composition comprising: a mixture of:an FR-4 epoxy resin; a crosslinking agent comprising a copolymer ofstyrene and maleic anhydride (SMA), and; at least 5% by weight of aco-crosslinking agent selected from the group consisting of a brominatedbisphenol A (BPA), brominated bisphenol A diglycidyl ether (BPADGE), andmixtures thereof.
 23. The laminate of claim 22 wherein the amount ofresin copolymer used will give an anhydride and aromatic hydroxyl group:epoxy group equivalency ratio of between 90 and 110%.
 24. The laminateof claim 22 wherein the resin includes at least 10% by weight oftetrabromobisphenol A and at least 10% by weight tetrabromobisphenol Adiglycidyl ether.
 25. The laminate of claim 22 wherein the resinincludes at least 10% by weight of tetrabromobisphenol A diglycidylether.
 26. The laminate of claim 22 wherein the resin is partiallycured.
 27. The laminate of claim 22 wherein the resin is fully cured.28. A printed circuit board including one or more laminate layerswherein at least one of the laminate layers is the laminate of claim 22.